Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are two prevalent duplexing schemes used in wireless networks. FDD, which historically has been used in voice-only applications, supports two-way radio communication by using two distinct radio channels. Alternatively, TDD uses a single frequency to transmit signals in both the downstream and upstream directions.
For FDD systems, one frequency channel is transmitted downstream from a radio A to radio B. A second frequency is used in the upstream direction and supports transmission from radio B to radio A. Because of the pairing of frequencies, simultaneous transmission in both directions is possible. To mitigate self-interference between upstream and downstream transmissions, a minimum amount of frequency separation can be maintained between the frequency pair.
For TDD systems, a single carrier frequency channel is used to transmit signals in both the downstream and upstream directions.
Half Frequency Division Duplex (HFDD) provides a duplexing scheme in which a downstream user receives signals on one carrier frequency over a first period of time, and transmits signals on another carrier frequency over a second period of time.
It is desirable to have a TDD system implementation that is operable as an HFDD system.